The present invention relates to a hydrogen-stored electrode for use in a battery and a method for the manufacture thereof.
In fact, a battery using metal oxide such as Ni-OOH and AgO for the positive electrode and hydrogen for the negative electrode has already been proposed and promoted for early development. There are two kinds of such a battery. One is conventionally called the "high-pressure" type, which causes hydrogen generated during charge to be stored under-compression in a pressure-resistant container in the vapor phase. This however requires an air-tight container which should be perfectly resistant to high pressure, thus creating a problem for development on a commercial basis. The other is the "low-pressure" type, which causes hydrogen generated during charge to be stored as a hydro-metallic compound in a hydrogen-stored alloy. Therefore, in principle, the actual pressure resistance needed for the container is determined by the equilibrium dissociation pressure, and yet, it does not require resistance against high pressure, thus providing more potential for early development than the other so-called "high-pressure" type. Actually, two methods are available for manufacturing an electrode (specifically, the negative electrode) using a hydrogen-stored alloy.
The first of these two methods is as follows. First, a hydrogen-stored alloy is compounded by means of a high-frequency furnace for example, and then reduced to powder by mechanical means. To the powder is added alkali-resistant organic high polymers, such as polyethylene or polytetrafluorethylene, which function as cross-linking agents. In addition, some selective conductive powder such as carbon, copper, or nickel may be added as required. The prepared mix is then subjected to compression molding to produce a conductive current collector such as punched metal or foamed metal for example. The molded conductive material is then thermally treated by applying a temperature close to the melting point of the cross-linking agent in an atmosphere of inert gas. As a result, an electrode is produced through such a relatively simple process. Nevertheless, since the hydrogen-stored alloy employed for this electrode was not activated in the initial stage, immediately after the electrode is completed, normally, there is no hydrogen content at all that can be electrochemically made available. In other words, immediately after its birth, no capacitance is provided for such an electrode produced by the above process. It will be provided with a certain capacitance only after more than 10 rounds of charge and discharge have been applied. In addition, such an electrode cannot contain sufficient capacitance throughout its service life.
There is a still further problem to solve. Since the hydrogen-stored alloy is subjected to compression molding without hydrogen content, when hydrogen content is stored during the following charge, the electrode expands its own volume by 10% to a maximum of 20%. As a result, the powdered alloy components drop from the molded piece, while the electrode shape becomes deformed. In addition, the capacitance value itself sharply decreases.
The second method is the following. After mechanically reducing the compounded hydrogen-stored alloy, the powder is stored in a container so that reaction can take place with hydrogen. While heating the interior of the reactive container, vapor is decompressed and discharged.
Next, the content of the reactive container is cooled at room temperature, and then high-pressurized hydrogen gas is fed into the reactive container, causing the hydrogen-stored alloy to absorb hydrogen. Next, the content of the reactive container is again decompressed and discharged to release hydrogen from the hydrogen-stored alloy. Then, high-pressurized hydrogen gas is again fed into the reactive container to allow the hydrogen-stored alloy to absorb hydrogen again. After fully activating the hydrogen-stored alloy by repeating these procedures several times, the activated hydrogen-stored alloy is drawn out of the reactive container, and then, as was done in the first method described earlier, the hydrogen-stored alloy is blended a selected cross-linking agent. Finally, by applying compression molding and thermal treatment to the mix, an electrode can be made up. Note that activation of the hydrogen-stored alloy provides an effect in which a sufficient volume of hydrogen can be easily stored in and discharged from the hydrogen-stored alloy, thus enabling the alloy to remain in a specific condition so that these serial operations can be done repeatedly and continuously. Since the second method causes the hydrogen-stored alloy to be fully activated, the resultant electrode is provided with sufficient capacitance and can be offered for use immediately after being made up. In addition, since the hydrogen-stored alloy has been compression molded in the hydrogenated (activated) condition, i.e., in the expanded condition, an electrode made of this alloy does not cause dissociation of the powder alloy nor deformation of of the electrode throughout the ensuing charge and discharge cycles. An electrode made of this alloy can maintain a specific capacitance provided during the production stage. Despite such advantageous features, the embodiment of the second method requires high-procedure-resistant reactive containers and other equipment during the manufacturing process. Moreover, since the second method also deals with highly pressurized hydrogen gas, large-scale production facilities should be employed, thus unavoidably involving complex operations. Furthermore, since the activated hydrogen-stored alloy contains extremely active elements, ensuing processes should be executed in the inert gas atmosphere. This not only hinders the mass production potential, but also increases production costs. As described earlier, although the first method features a simple production method, it cannot provide an electrode with sufficient electrical characteristics. Conversely, despite satisfactory characteristics of the resultant electrode, the second method still involves an extremely complex production processes. As a result, neither of these methods has been found satisfactory for the manufacture of electrodes.